Research addressing the human response to radionuclear events is urgently needed for developing effective medical countermeasures. The unpredictable nature as well as the complexity of the injuries from a nuclear blast makes this a challenging world health problem. Despite the public health danger, there are limited treatment options for protecting the potentially high numbers of individuals that might be exposed to varying radiation or radiation with combined trauma. If an event does occur, an immediate concern for survivors will be the development of opportunistic infections because radiation and injury damage the immune system making it difficult for patients to resist infections. We hypothesize that giving immune response modifier (IRM) treatment after radiation injury or radiation combined injury (RCI) will protect injured individuals from the complications of these complex injuries by restoring immune system homeostasis. We developed an outbred mouse model to characterize the effects of radiation and RCI on the immune system and to test several IRM strategies as treatments for restoring immune system function after radiation and RCI. During the course of this project, we discovered that a CpG-containing oligodeoxynucleotide (ODN) that mimics bacterial DNA could signal a highly protective immune response in mice when given at 1 day after radiation injury or RCI. CpG ODNs activate cells by stimulating Toll-like receptor 9 (TLR9) receptors and their biological activities depend on their nucleic acid sequence and structure. We found that a specific CpG ODN sequence, called CpG ODN 2336, protected mice from post-injury infection and sepsis even in mice exposed to 6 Gy whole-body radiation doses with combined burn injury. This project will use this discovery to test and advance the potential of using CpG ODNs as broad protective medical countermeasures for radiation and radiation combined injury. In Aim 1, pharmacokinetic studies will be performed to determine effective dose ranges and timing for subcutaneous (s.c.) delivery of CpG ODN 2336 to mice post radiation. This aim will use a systems immunology approach to measure immune cell regeneration and anti-microbial immunity to judge effective treatment doses. Toxicity studies will also be performed. Aim 2 studies will delineate the biological activities and mechanisms-of-action for CpG ODN treatment effects on the hematopoietic and immune system. Aim 3 will investigate and compare species specificities of CpG ODN activation properties on human and mouse immune cells. The translational potential of CpG ODNs as protective treatments after radiation injury will be advanced by the results of these comparative studies. Aim 4 experiments will test the effectiveness of CpG ODN treatments in radiation combined injury (RCI) mouse models. The RCI models will include radiation with burn injury or head injury. Collectively, the results of these studies will provide pre-clinical data for translating CpG ODNs as effective medical countermeasures for treating to the variety of injuries that could occur following a nuclear event.